Conventional tape drive data storage systems employ various error correction and recovery methods to detect and correct data errors which, if left unresolved, would compromise the integrity of information read from or written to the magnetic tape media. Events which can lead to data errors include defects on the media, debris between the tape head and the media, and other conditions that interfere with head/media data transfer operations.
Error correction and recovery may be thought of as two distinct operations that are employed at different stages of error processing. Error correction is conventionally implemented using error correction coding (ECC) techniques in which random host data to be placed on a tape medium is encoded in a well-defined structure by introducing data-dependent redundancy information. The presence of data errors is detected when the encoded structure is disturbed. The errors are corrected by making minimal alternations to reestablish the structure. ECC error correction is usually implemented “on-the-fly” as data is processed by the tape drive apparatus. The well-known Reed-Solomon code is one cyclic encoding scheme which has been proposed for ECC error correction. Other encoding schemes are also known in the art.
Error recovery occurs when ECC error correction is unable to correct data errors. The error recovery process usually requires stopping the tape and reprocessing one or more data blocks in which an error was detected. Thus, the tape drive system usually attempts to re-read the data and possibly attempts to change certain parameters that may enable the data block to be recoverable. In many instances there are portions of the data block that may be recoverable on a retry, but may not be sufficient to recover the data. On subsequent retries other areas of the data may be recoverable. Since all of the data or at least the minimum amount necessary for error correcting codes to be successful is not met, the block remains unrecoverable.
Therefore, it is desirable to both retrieve data that conventional tape drives may never retrieve in a timely manner.